This study is concerned with the requirements of sterol for growth and differentiation of parasitic nematodes in axenic culture. During the past year observations have been made on the lesions which develop in larvae cultivated in the absence of exogenous cholesterol using light and electron microscopy. Vitamin B12 is also being used as a biochemical marker to analyze systematic relationships of helminths and to re-evaluate previously proposed evolutionary schemes, particularly for the Cestoda.